1. Field of the Invention
The present invention generally relates to an inspecting or examining device, and more particularly, to an inspecting device for inspecting printed state of cream solder printed on a printed circuit board for mounting thereon, at high density, very small electronic parts (1005 square chip and the like), narrow lead pitch IC, etc., as employed in headphone stereophonic appliances, cordless telephones, video cameras and the like.
2. Description of the Related Art
In the case where mounting of electronic parts onto the surface of a printed circuit board is to be automated, it is normally so arranged to print the cream solder on each specific position on a printed circuit board, and then, to temporarily fix the electronic parts on said printed circuit board by viscosity of the cream solder for subsequent soldering through a reflow furnace. Thus, it has been conventional practice to check the state of soldering of the printed circuit board after completion of the soldering process by the reflow furnace. However, due to the requirement for mounting at a higher density onto the printed circuit board, electronic parts to be soldered at the reverse face side, for example, LSI (large scale integrated circuit) having J leads, etc. have come into actual use, and in this case, it becomes difficult to effect inspection after completion of soldering. Moreover, the state of soldering of such electronic parts is largely affected by the state of printing when the cream solder is printed on the printed circuit board. Therefore, it becomes necessary to examine the printed state of the cream solder before the soldering process by the reflow furnace, and for this purpose, inspectors were distributed for effecting visual inspection.
However, such visual examination by the inspectors tends to increase the burden for the inspection, with a low working efficiency.
For examining the printed state of the cream solder, there has been conventionally employed an inspecting device arranged to project slit light onto a printed circuit board for picking up or photographing the projected pattern, so as to judge defects of soldering, positional deviation, etc. from the image signal obtained thereby. The known inspecting method based on the slit light projection as referred to above is higher in a detecting resolution as the interval for the slit light becomes small, while the detection speed is higher as the interval becomes large. In other words, accuracy and speed are in a perfect trade-off relation, and furthermore, there is such a disadvantage that, in the above inspecting method by the slit light projection, the coordinate values are available only on the slit.
Additionally, in the case where the deviation amount of the slit light, depending on a three-dimensional configuration of the plane to be measured, is detected, since one-slit tends to overlap a neighboring slit in some cases, a correct deviation amount can not be obtained. Thus, effecting retry or some countermeasures in spatial coding such as coloring the slit, etc. are required. Accordingly, in the conventional printed state inspecting device, there has been a limit to the improvement of accuracy, and reduction of measuring time.